1. Field of the Invention
The present invention relates to semiconductor laser diodes. More particularly, it relates to semiconductor laser with a ridge-waveguide structure and an electro-absorption modulator integrated DFB laser carrying the same.
2. Description of the Related Art
Recently, the demands for greater transmission capacities and increase in transmission speed are rapidly growing with the explosive increase in the Internet users, and it is considered that optical communications will play an important role in the future, too. Semiconductor lasers are widely used as light sources for optical communication systems. With the spread of optical communications, not only increase in modulation speed but also reduction in power consumption and reduced costs are strongly required. Various type of semiconductor lasers having different emission wavelengths are used for suitable applications, especially for suitable transmission distances. For short-reach applications whose transmission distance is 10 km or less, a directly modulated semiconductor laser with 1.3 μm wavelength band is mainly used.
In the case of the directly modulated lasers, because a optical module is realizable with a simple constitution, its power consumption is small. Since the number of parts consisting of the module can be reduced, reduction in cost is also possible. The transmission speed of such optical modules in practical use is now becoming 10 Gb/s. As directly modulated semiconductor lasers, there are a vertical-cavity surface-emitting laser (VCSEL) and an edge-emitting laser (EEL) Owing to a cavity length of several μm, VCSEL operates with an operation current of about several to ten mA, and, thus, its power consumption is small. Further, a laser beam is emitted at vertical direction to a substrate. So, the sorting of laser chip is possible in the state of wafers before cleaving process. Thus, VCSEL is also good for cost reduction. However, when using a VCSEL with 1.3 μm wavelength band which is suitable for a standard single-mode fiber, in case of the transmission distance of several km, optical output power of several mW required for transmission cannot easily be obtained. This is due to small aperture region of about 5 μm in VCSEL to emit a single-mode laser beam.
The cavity length of the conventional EEL is limited to about 200 μm. Therefore, for the high-speed operation of 10 or more Gbits/s, typical drive current is about 60 mA or more. As a result, in case of conventional EEL, it is difficult to further reduction in the power consumption. Therefore, as a new directly modulated type light source taking its place, a short cavity type of a laser described in Patent document 1 (JP-A No. 2007-5594) is proposed. In order to achieve both the low-current operation, which is the key to a low power consumption, and the optical output of several mW necessary for transmission distance of several km, the length of an active layer region is shortened. Further, the cavity length of a laser is set to a value within a range of from 10 to 100 μm which is an intermediate value of those of the conventional VCSEL and EEL. Thus, assuming that the upper limit of a drive current is set to about 80 mA, it is shown by calculation that a maximum value of the relaxation oscillation frequency is obtained under such cavity-length conditions. At the same time, there are also proposed an cost-effective structure which has slant reflecting mirror for converting the direction of a laser beam to be that of the surface emitting type and which has a lens for focusing the laser beam.
FIG. 1 shows a bird's eye view of a short cavity type of a laser. The laser structure is of a distributed Bragg reflector (DBR) type. FIG. 2 shows a cross-sectional structure of the short cavity type of a laser. Numeral 101 and 201 are lower electrodes; 102 and 202 are n-InP substrates; 103 and 203 are HR coating films; 104 and 204 are active layers; 105 and 205 are p-InP cladding layers; 106 and 206 are contact layers; 107 and 207 are upper electrodes; 108 and 208 are diffraction gratings; 109 and 209 are InGaAsP layers of a DBR region; 110 and 210 are slant reflectors; and 111 and 211 are back-surface lenses. Here, lengths of the active layers 104 and 204 are set to values within a range of from 10 μm to 100 μm. A distributed-feedback (DFB) type may also be applied to the present laser. In that case, the diffraction grating is formed above or under the active layer, and InP layers are often used instead of the InGaAsP layers 109 and 209 of the DBR region. This InP layer may also be formed at the same time that the p-InP cladding layer is formed. In such a case, pn junction of InP is may formed. At this time, as described in Patent document 2 (JP-A No. 2004-235182), it is preferable to form a reflecting mirror composed of semiconductor layers having two kinds of different refractive indices under the active layer.
With respect to structures of semiconductor laser diodes, there are roughly two kinds, that is, a ridge waveguide (RWG) structure and a buried-hetero (BH) structure. This is the same for the previously described short cavity type of a laser. FIGS. 3A and 3B show a cross-sectional view perpendicular to the mesa stripe direction and a cross-sectional view parallel to the mesa stripe direction of the RWG structure and the BH structure of the DBR-type lasers, respectively. In FIG. 3A, numeral 301 is an n-InP substrate; 302 is an active layer; 303 is a p-InP cladding layer; 304 is a contact layer; 305 is a diffraction grating; 306 is an InGaAsP layer in a DBR region; 307 is a slant reflector; and 308 is a back-surface lens. Further, in FIG. 3B, numeral 309 is an n-type InP substrate; 310 is an active layer; 311 is a p-InP cladding layer; 312 is a contact layer; 313 is a diffraction grating; 314 is an InGaAsP layer in the DBR region; 315 is a slant reflector; 316 is a semi-insulating InP buried layer; and 317 is a back-surface lens In this case, for the simplicity of the explanation, electrodes and HR coating films are not illustrated. Also, a region including the portion of the slant reflector at the edge of the active layer is generally called a window region. In the conventional EEL, a window region is sometimes formed to suppress the interference between the optical feedback reflected at a cleaved facet and a laser beam.
In the RWG structure shown in FIG. 3A, when forming a mesa structure having a width of several μm by etching the upper cladding layer 305, etching is stopped above the active layer 302. Since the active layer portion is not etched, excessive damage to the active layer is not caused, which is advantageous in terms of highly reliable operation of the laser. On the other hand, in the BH structure shown in FIG. 3B, when forming a mesa, etching is performed deeply enough to a portion below the active layer 310. Further, a high-insulating semiconductor layer 316 is again buried in both sides of the mesa including the active layer. Therefore, sides of the active layer may be damaged during mesa etching, and its quality may be degraded. Particularly, in the active layers including an InGaAlAs material for 1.3 μm wavelength band containing high-content Al atoms, a chemically robust oxides is formed at the side of the active layer after the mesa etching process. As a result, a better buried hetero epitaxial growth is disturbed. Therefore, in order to overcome this difficulty, a special treatment for the side of the active layer is often necessary just before a crystal growth, and, thus, it is not easy to realize highly reliable operation of the laser.
Further, as described earlier, when integrating slant reflectors in the window region, in order to increase its reflective efficiency of the laser beam and allow the laser beams to focus on the lens effectively, a precise control of the etching angle of the slant reflector is indispensable. Since the slant reflectors are formed by dry etching or wet etching, when considering the controllability of the etching angle, it is preferred that a surface of the window region before etching should be flat. When comparing the RWG structure with the BH structure from this point of view, the surface of the window region of RWG structure in which the p-InP cladding layer 303 is re-grown on a surface where the active layer 302 and the InGaAsP layer 306 are smoothly joined in lateral direction, is more flatter than that of the BH structure in which the semi-insulating InP buried layer 316 is re-grown on a surface with a large thickness difference of more than 2 μm formed by mesa etching. It is preferred that the surface is flat in terms of manufacturing an laser with a high yield in the manufacturing process to be followed. Therefore, for the short cavity type of a laser, when comparing the RWG structure with the BH structure in terms of reliable operation and surface flatness, it is understood that the RWG structure is more advantageous than the BH structure.